As a working machine provided with a load sensing system, there has been a back hoe described in Patent Literature 1.
In this back hoe, there are equipped a plurality of hydraulic actuators and a main pump composed of a variable displacement hydraulic pump for supplying pressure oil to these hydraulic actuators.
The load sensing system is provided with: direction switching valves which are provided in correspondence with the respective hydraulic actuators to control a supply direction of the pressure oil discharged from the main pump and supply the pressure oil to the hydraulic actuators; and pressure compensation valves provided in correspondence with the respective direction switching valves functioning so as to keep a front and rear differential pressure of each of the direction switching valves.
Further, the load sensing system is provided with a flow rate control part for controlling the main pump, in addition to the variable displacement main pump, direction switching valves and pressure compensation valves, and further includes a PPS transmission line for transmitting a discharge pressure of the main pump as a PPS signal pressure to the flow rate control part and a PLS transmission line for transmitting a maximum load pressure among load pressures of the hydraulic actuators as a PLS signal pressure to the flow rate control part.
The flow rate control part controls the discharge pressure of the main pump so as to maintain the differential pressure obtained by subtracting the PLS signal pressure from the PPS signal pressure to be a constant pressure.
In the case where the plurality of hydraulic actuators are operated, the load sensing system diverts the discharge flow rate of the main pump so as to supply the pressure oil to each of operated hydraulic cylinders by a quantity corresponding to an operated amount regardless of difference in value of loads acting on the operated hydraulic actuators.
The pressure compensation valve is provided with a load pressure flow passage for introducing a load pressure of a hydraulic actuator corresponding to the pressure compensation valve and outputting the load pressure to the PLS transmission line.